A method for preparing metal matrix composites containing modified polytetrafluoroethylene

ABSTRACT

Polytetrafluoroethylene, sold under the trademark Teflon, is modified before being blended with metal powder and formed into a composite. Modification is accomplished by heating the Teflon for 1 to 2 hours at 1,000*-1,100*F and then for 3 to 3.5 hours at 650* to 750*F. Metal matrix composites containing modified Teflon as a filler are useful as retainer materials for bearings which are subjected to high temperature operating conditions.

Unite States aten Boes 1451 July 18, 1972 [54] METHOD FOR PREPARDIG METAL [55] References Cited gigERIX SOMPOSITES CONTAINING UNHED STATES PATENTS 3,273,977 9/1966 Davis ..260/92.1 S POLYTETRAFLUOROETHYLENE 2,964,476 12/1960 Coad ..252/ l2 [72] Inventor: David J. Boes, Monroeville, Pa. 2,294,404 1942 P1811881 8t l 2 2,933,536 4/1960 Wall et al ...260/92.l S Ass1gnee= The United States of America as 3,432,511 3/1969 Reiling ..260/92.1 s

represented by the Secretary of the Air Force Primary Examiner-Robert F. White Assistant Examiner-Jefi'ery R. Thurlow [22] 1969 Anomey--Harry A. Herbert, Jr. and Cedric 11. Kuhn 21 App]. No.: 887,730

[ ABSTRACT Poiytetrafluoroethylene, sold under the trademark Teflon, is [52] Cl 7 modifi' ed before being blended with metal powder and formed into a composite. Modification is accomplished by heating the [51] Int. Cl ..C08f 3/24, Cl0m 5/10, Fl6d 69/00 Teflon for l to 2 hours at IOOOLUOOQF and the" for 3 to 3'5 [58] Field of Search ..264/127, 126, 122, 331, 319, hours at 50 to 750 Meta] matrix composites containing modified Teflon as a filler are useful as retainer materials for bearings which are subjected to high temperature operating conditions.

2 Claims, No Drawings BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is in the field of solid lubricants.

2. Description of the Prior Art Self-lubricating composites containing Teflon either as a filler or as the matrix have been used extensively as load bearing surfaces in applications where lubrication by conventional oils and greases is unsatisfactory or impractical. When applied as retainers in otherwise unlubricated ball bearings, composites containing Teflon polymer have provided long term and reliable operation in ultrahigh vacuum environments and at cryogenic temperatures. However, such composites have found little use as retainers in situations where temperatures are high (exceeding about 450F). This is due to the fact that thermal degradation causes polytetrafluoroethylene-containing composites to lose their high load-bearing capacity and desirable friction-wear characteristics. To achieve greater temperature capabilities, it has been necessary to substitute more thermally stable resins, such as polyimides, for polytetrafluoroethylene. This substitution hasafl'ected the ultimate properties of the final product. For example, the substitution of polyirnide resin for Teflon in a silver matrix composite causes a twofold increase in friction and more than a tenfold increase in wear under bearing presures of 500 psi.

In order to eliminate the problem discussed in the preceding paragraph and other problems associated with the use of substitute resins for Teflon, research has been carried out on methods of making Teflon suitable for high-temperature use.

SUMMARY OF THE INVENTION It has now been found that if polytetrafluoroethylene, which is commonly sold commercially under the trademark Teflon, is subjected to heating at about l,0O to l,lO0F for from 1 to 2 hours and then further heated for at least 3 and preferably not more than 3.5 hours at a temperature of from 650 to 750F, its high temperature properties are greatly modified. The two heating periods (l,000-l,l00F for 1 to 2 hours followed by 650-750F for 3 to 3.5 hours) may either be carried out with no interruption between (by transferring the molten material from one fumace to another) or by allowing the polytetrafluoroethylene to cool between heating steps. The modification of the high temperature (above about 450F) properties of the polytetrafluoroethylene results in a product which may be used as filler or as matrix material in high temperature load-bearing composites. The composites hereinafter disclosed have silver or silver/mercury amalgam matrixes with modified polytetrafluoroethylene and molybdenum diselenide as fillers. The composites show greatly improved lifetimes over prior art composites when fabricated into bearing retainers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Wear tests performed on self-lubricating composites described in the following examples were made by rubbing a 0.25 inch face with a rotating tool steel disc moving at 2,550 ft/min. A 3-pourid load was applied to each test specimen by means of a lever and dead weight assembly to press it against the rotating disc. Wear was observed as scar width in millimeters. Friction was determined at 5-minute intervals by observing the strain gauge deflection on an SR-4 strain indicator as transmitted from a dynamometer attached to the test apparatus. Temperature was monitored with a thermocouple.

EXAMPLE I Two hundred grams of powdered polytetrafluoroethylene (100 mesh) were placed in a porcelain dish, inserted in a furnace, and heated at a temperature of 1,100F for 1.5 hours. The material was then removed from the furnace and, still in its molten condition, placed in a second furnace where it was heated for 3 hours at 700F. During the second heating period, the polytetrafluoroethylene changed color from brownishgrey to pure white. The color change was apparently due to the removal of certain volatile and carbonaceous impurities. When removed from the furnace after the second heating, the polytetrafluoroethylene was in a molten, viscous state. Upon cooling, the modified Teflon was ground in a Wiley mill to a 20-100 mesh particle size.

EXAMPLEII Polytetrafluoroethylene modified according to example I was blended with silver powder (325 mesh) and molybdenum diselenide powder according to the following proportions.

Component Volume Silver (325 mesh) Modified Polytetrafluoroethylene 20% Molybdenum Diselenide 10% Friction Coefiicient Wear mm Scar Modified polytetrafluoroethylene was further subjected to melting point detemrinations and, unlike unmodified polytetrafluoroethylene exhibited a sharp melting point at 590F. When incorporated into a composite in the abovedescribed manner it provided a thin, fluid film of resin on the metal components with which it was in contact. Thus, dynamic friction coefficients and wear of the polytetrafluoroethylene as well as bearing components were reduced by modification of the Teflon.

EXAh/[PLE III Polytetrafluoroethylene modified by the method of Example I, silver (325 mesh), and molybdenum diselenide powder were blended according to the following proportions.

Component Volume Silver (325 mesh) Modified Polytetrafluoroethylene 5% Molybdenum Diselenide 15% The powder mixture was formed into a composite by heating under compression as described in Example II. Tests on the composite yielded the following results.

Friction Coelficient Wear mm Scar 600F 0.09 3.5 900"F 0.44 l 1.0

EXAMPLE IV A mixture of 70 weight percent silver and 30 weight percent mercury were used as a matrix for modified polytetrafluoroethylene and molybdenum diselenide powder according to the following proportions.

Component Volume 70 wt.% Ag/30 wt.% Hg 70% Modified Polytetrafluoroethylene 20% Molybdenum Diselenide 10% Initial wetting of the silver by the mercury was accomplished by blending in a ball-mill. Blending was accomplished by V- blending (as were the blends of Examples I and II). The blend was formed into a composite by compressing in a die at 50,000

Wear mm Scar Friction Coefficient 600F 0.04

The following table presents functional test data from several 204 size ball bearing experiments which provide a comparison between the operating life of bearing systems equipped with retainers using unmodified polytetrafluoroethylene as opposed to those using modified polytetrafluoroethylene. The experiments were performed at 600F and at a speed of 10,600 rpm. All bearings carried a combined load of 50 lbs. thrust/50 lbs. radial. It will be noted that the bearing systems equipped with silver matrix retainers filled with modified polytetrafluoroethylene provided an improvement in operating life of about 160 percent over identical retainers employing the unmodified polymer. The data also indicate the substantial improvement in life that is realized by using the self-lubricating retainers of this invention in lieu of high temperature grease.

TABLE FUNCTIONAL 204 SIZE BALL BEARING TESTS COIVIPARING THE USE OF UNMODIFIED AND MODIFIED PTFE FILLED RETAINERS 600F 10,600 rpm 50 lbs. thrust/50 lbs. radial Retainer Composition: 70% Ag 20% PTFE MoSe (vol.

Run

Polytetrafiuoroethylene Life The above examples give specific times and temperatures for the modification procedure for polytetrafluoroethylene. Experimentation has shown that the modification steps may be carried out within the time and temperature ranges of from I to 2 hours at l,000 to l,l00F and from 3 to 3.5 hours at 650 to 750F without detracting from the excellent high temperature qualities of the final modified polytetrafluoroethylene. Experiments with composites utilizing only modified polytetrafluoroethylene with silver or silver/mercury amalgams in lieu of the three-component systerm disclosed gave poor results. However, it is within the scope of the invention to use other solid lubricant powders such as molybdenum disulfide in lieu of the molybdenum diselenide disclosed. Experiments also showed that there is no difference in results if the polytetrafluoroethylene is allowed to cool between the two heating periods of its modification rather than being transferred from one furnace to another while in its molten condition as disclosed in Example I.

I claim:

1. The method of manufacturing a self-lubricating composite article comprising the steps of:

a. modifying polytetrafluoroethylene by heating it for about 1 to 2 hours at about l,000 to l, l00F and thereafter for about 3 to 3.5 hours at about 650 to 750F;

. allowing the modified polytetrafluoroethylene to cool and thereafter grinding it into a powder;

0. blending a mixture consisting of l520 volume percent of the modified polytetrafluoroethylene with -70 volume percent silver powder and 15-10 volume percent molybdenum diselenide to form a blended powder mixture; and

d. forming the blended powder mixture into said composite article by heating said mixture to a temperature of from 500 to 520F for about 15 minutes while compressing said mixture under a pressure of about 100,000 psi.

2. The method of manufacturing a self-lubricating composite article comprising the steps of:

a. modifying polytetrafluoroethylene by heating it for about 1 to 2 hours at about 1,000 to 1,100F and thereafter for about 3 to 3.5 hours at about 650 to 750F;

b. allowing the modified polytetrafluoroethylene to cool and thereafter grinding it into a powder;

0. blending a mixture consisting of 20 volume percent of the modified polytetrafluoroethylene powder and 10 volume percent molybdenum diselenide powder with 70 volume percent of silver-mercury amalgam powder; and

d. forming the blended powder mixture into said composite article by heating said mixture to a temperature of from 500 to 520F for about 15 minutes while compressing said mixture under a pressure of about 50,000 psi. 

2. The method of manufacturing a self-lubricating composite article comprising the steps of: a. modifying polytetrafluoroethylene by heating it for about 1 to 2 hours at about 1,000* to 1,100*F and thereafter for about 3 to 3.5 hours at about 650* to 750*F; b. allowing the modified polytetrafluoroethylene to cool and thereafter grinding it into a powder; c. blending a mixture consisting of 20 volume percent of the modified polytetrafluoroethylene powder and 10 volume percent molybdenum diselenide powder with 70 volume percent of silver-mercury amalgam powder; and d. forming the blended powder mixture into said composite article by heating said mixture to a temperature of from 500* to 520*F for about 15 minutes while compressing said mixture under a pressure of about 50,000 psi. 